An anodizing treatment has been performed as a method for improving the corrosion resistance of aluminum metal or an aluminum alloy (hereinafter, the aluminum metal or aluminum alloy is also referred to as an “aluminum-based metal”). The anodizing treatment is a method in which aluminum is oxidized to form an oxide film on the surface, and it has been used in a wide variety of applications ranging from household goods to parts for industrial use.
However, an anodic oxide film does not protect a defect portion, and hence there is a problem in that when a defect reaching the aluminum base material is created in the use of a product, corrosion in the defect portion proceeds and spreads to the surroundings. For example, the corrosion resistance of an aluminum exterior part of an outboard engine, which is used in a harsh corrosive environment, is improved by providing an anodic oxide film and a coating. However, the aluminum exterior part is easily damaged by contact with sand or rock, and corrosion proceeds at an accelerating pace when the defect portion comes into contact with seawater. The corrosion causes not only degraded appearance, but also operation failure due to the formation of a hole penetrating through the part. For this reason, there has been a need for a method for inhibiting the progress of corrosion even when a defect is created.
Besides the anodizing treatment, methods using zinc, which has sacrificial corrosion protection properties, have been known as methods for improving the corrosion resistance. The methods include a method in which the surface of an aluminum-based metal serving as a base material is coated with a coating material containing zinc powder such as a zinc-rich paint, and a method in which the surface is plated with zinc. In these methods, even when the base material is defective, zinc serves as a sacrificial anode and is dissolved in a self-sacrificial manner so as to exhibit a sacrificial corrosion protection function. In this manner, the corrosion protection in a defect portion is inhibited.
Moreover, methods for inhibiting corrosion in a defect portion have been known in which a coating material containing hexavalent chromium having a self-repairing function is applied onto the surface of a base material, or a film having a self-repairing function is formed on the surface of a base material to improve the corrosion resistance. A general coating material or film having a self-repairing function inhibits the corrosion in the defect portion in such a manner that the coating material component is dissolved in response to an external factor such as ultraviolet light and flows to the defect portion to cover the defect portion.
In addition to these methods using zinc or hexavalent chromium, methods using alkali metals such as lithium and sodium are known. For example, a method is known in which, to conduct a corrosion inhibition treatment on the surface of an aluminum-based metal, an aqueous solvent in which an alkali metal silicate, an oxidizing or reducing inorganic compound, and an inorganic phosphoric acid salt are dissolved or the like is applied onto the metal surface to form a corrosion inhibition film thereon (PTL 1). In this method, the corrosion is inhibited in such a manner that the insoluble silicate in the corrosion inhibition film blocks an aqueous solution containing a corrosive factor such as oxygen or chloride ions from reaching the metal. In addition, when a defect reaching the aluminum-based metal is created, the water-soluble silicate and the inorganic compound dissolved in the corrosion inhibition film migrate to the defect portion and causes formation of an aluminum passivation film to protect the damaged portion.
In addition, a method is also known in which a treated film is formed on a surface of an aluminum-based metal by bringing the aluminum-based metal into contact with an aqueous alkaline solution containing a lithium compound (PTL 2).